An internal combustion engine has been widely used in a vehicle, a ship, a small generator, or the like and an attempt to improve efficiency of the internal combustion engine has been continuously conducted. In the internal combustion engine, a large quantity of heat is generally discharged as waste heat. For this reason, a waste heat recovery system for increasing the overall efficiency of the internal combustion engine by recovering the waste heat as energy has been developed.
The waste heat recovery system is configured to recover waste heat discharged from an engine as energy, convert the recovered energy into electric energy or mechanical energy, and utilize the converted energy in the engine of the vehicle, other accessories, or the like.
The waste heat recovery system is configured to include a Rankine cycle to very effectively recover the waste heat of the engine. The Rankine cycle includes a circulation path through which a working medium is circulated, in which the circulation path of the Rankine cycle includes a boiler (evaporator) configured to heat and evaporate the working medium by the waste heat (heat from exhaust gas and/or heat from EGR gas) of the engine, an expander configured to expand the gaseous working medium supplied from the boiler to generate rotational energy, an condenser configured to condense the working medium discharged from the expander, and a pump configured to circulate the working medium on the circulation path.
The expander is the most important part in the overall performance and durability of the waste heat recovery system and is directly exposed to highest temperature and pressure working medium within the waste heat recovery system and continuously rotated, and therefore is the most likely to be damaged.
Meanwhile, an expander for recovering waste heat that is currently under development is basically a turbine type expander. To prevent rotating parts, such as a bearing in a turbine that rotates at a high speed of 100,000 rpm or more, from being worn and damaged, there is a need to supply lubricating oil to the expander and the rotating parts such as a reducer connected to the expander while the waste heat recovery system is operated.
However, due to characteristics of the turbine type expander, some of the lubricating oil may be introduced into a working fluid conduit of the waste heat recovery system while being mixed with the working fluid. As such, if the lubricating oil is circulated while being mixed with the working fluid, the lubricating oil generates an oil film inside a heat exchanger, such that heat transfer efficiency may be reduced and the durability of the expander may be reduced due to a lack of lubricating oil.
To cope with the problem, a structure in which an oil separator is installed at a downstream side of the expander to separate the lubricating oil mixed with the working fluid and the separated lubricating oil is recovered to a lubricating oil tank is currently under development.
However, the lubricating oil recovered to the lubricating oil tank through the oil separator may be mixed with the working fluid, and therefore it is likely to introduce the working fluid into the lubricating oil tank. As such, if the working fluid introduced into the lubricating oil is not separated, the lubricating performance may be reduced and the working fluid may overflow the lubricating oil tank in excess of storage capacity of the lubricating oil tank.